1. Field of the Invention
The present invention relates to a novel planographic printing plate precursor, and more specifically, a planographic printing plate precursor which can be imagewise scanning-exposed by a laser ray based on digital signals and which has superior sensitivity and stain resistance.
2. Description of the Related Art
Planography is a printing method using a plate member having a lipophilic area which receives ink and an ink-repelling area (hydrophilic area) which does not receive ink but receives moistening water. At present, photosensitive planographic printing plate precursors (PS plates) have been widely used in planography.
As one of the PS plates, a plate wherein a photosensitive layer is formed on a support such as an aluminum plate has been practicable and widely used. Such a PS plate is imagewise exposed to light and developed to remove the photosensitive layer at a non-image portion, and printing is performed by utilizing the hydrophilicity of the support surface and the lipophilicity of the photosensitive layer at an image portion. In such a plate member, the support surface needs to be highly hydrophilic in order to prevent staining of the non-image portion.
Conventionally, the hydrophilic support or the hydrophilic layer used in planographic printing plate precursors are generally anodized aluminum supports or anodized aluminum supports treated with silicate to further improve the hydrophilicity thereof. Furthermore, research on hydrophilic supports or hydrophilic layers using such aluminum supports have been actively made. A support treated with an undercoat agent made of polyvinyl phosphonic acid and a technique using a polymer having a sulfonic acid group as an undercoat layer for a photosensitive layer, are known. Moreover, a technique using polyvinyl benzoic acid or the like as the undercoat layer has also been suggested.
With regards to a hydrophilic layer in the case of not using a metal support such as an aluminum support but using a flexible support such as a PET (polyethylene terephthalate) support or a cellulose acetate support, the following techniques are known: a technique of forming, on a PET support, a hydrophilic layer which contains a hydrophilic polymer and is cured with hydrolyzed tetraalkyl orthosilicate (see, for example, Patent document 1 (Japanese Patent Application Laid-Open (JP-A) No. 8-272087)), and a technique of forming a hydrophilic layer having a phase-separation structure composed of two phases, namely, a phase with of a hydrophilic polymer as a main component and a phase with a hydrophobic polymer as a main component (see, for example, Patent document 2 (JP-A No. 8-292558)), and other techniques.
These hydrophilic layers have higher hydrophilicity than conventional hydrophilic layers, and provide planographic printing plates capable of supplying printed matters having no stains at an initial stage of printing operations. However, when printing is repeated, problems, such as the hydrophilic layers peeling or the hydrophilicity thereof decreasing with the passage of time, occur. Thus, it has been desired to develop planographic printing plate precursors which are able, even under more harsh printing conditions, to supply a great number of printed matters having no stains, without hydrophilic layers of the planographic printing plates being peeled from their supports or the hydrophilicity of their surfaces being lowered. From a practical viewpoint, it is required to improve the hydrophilicity still more in the present situation.
With regards to printing plates for computer-to-plate systems, which have been remarkably progressed in recent years, much research has been made. In particular, development-free planographic printing plate precursors, which are set to a printing machine for printing without being developed after being exposed to light, have been researched in order to make printing-processing more efficient and solve the problem of waste liquid disposal. As a result, various methods have been suggested.
One of the methods for removing the disposal step is a method called on-machine development, which comprises a step of fitting an exposed printing precursor to a cylinder of a printing machine; and a step of supplying moistening water and ink thereto while rotating the cylinder, thereby removing the non-image portion of the printing precursor. That is, this is a method of exposing the printing precursor to light; then setting the plate, as it is, to a printing machine; and completing development in the course of an ordinary printing process.
It is necessary that a planographic printing plate precursor suitable for such on-machine development has a photosensitive layer soluble in moistening water and ink solvent and further has good bright-room-handling performance suitable for being developed on a printing machine located in a bright room.
As a printing plate precursor for which no developing step is necessary, there is known a non-processed printing plate precursor in which a crosslinked hydrophilic layer is formed on a support, the crosslinked layer containing microencapsulated heat-meltable material (see, for example, Patent document 3 (WO No. 94/23954 pamphlet)). In this printing plate precursor, the microcapsules collapse by the action of heat generated in the area exposed to a laser and then lipophilic material in the capsules is melted out so that the surface of the hydrophilic layer is made hydrophobic. This printing plate precursor does not need to be developed, but the hydrophilicity or the durability of the hydrophilic layer deposited on the support is insufficient, thereby resulting in a problem wherein, as printing using the plate is repeated, the non-image portions in printed matters gradually become more stained.